AA

'A'a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers.

Agglutinate

A welded pyroclastic deposit; term is commonly used for deposits of bombs fused while hot and viscous. Agglutinate typically occurs in spatter cones.

Alluvium

A general term for deposits made up of a mixture of loose rocks that were produced by mechanical means (detritus). Deposits of alluvium are typically formed as a result of flooding or transportation in a stream or river. e.g. an alluvial fan.

Ash-flow tuff

Back-arc basin

The area behind a volcanic arc and within the overriding plate in a subduction zone environment.

Basalt

Volcanic rock (or lava) that characteristically is dark in color (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium. Basaltic lavas are more fluid than andesites or dacites, which contain more silica.

Basaltic andesite

Base surge

A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity from the base of a vertical eruption column. Can accompany phreatomagmatic eruptions.

Batholith

A large mass of rock formed by magmatic processes that has more than 100 km2 (40 mi2) of surface exposure and no known floor.

Benchmark

A location on the surface of the earth at a specific latitude and longitude that is used as a point of reference. A marker is often installed at a benchmark location, so that the same exact point can be revisited over and over again to compare and assess changes through time.

Bishop Tuff

Block and ash flow

Bomb

Volcanic bombs are lava fragments that were ejected while viscous (partially molten), and blocks are solid rock fragments; both are larger than 64 mm in diameter and are ejected during an explosive eruption.

Breadcrust Bomb

A breadcrust bomb is a volcanic bomb with a cracked and checkered surface, sometimes resembling the surface of a loaf of bread. The cracks develop when the outer surface of a partially molten lava fragment cools to form a brittle surface and then subsequently cracks as the hot interior expands due to the continued growth of gas bubbles.

Caldera

A large basin-shaped volcanic depression with a diameter many times larger than included volcanic vents; may range from 2 to 50 km (1 to 30 mi) across.

Cinder cone

A conical hill, often steep, formed by accumulation of solidified fragments of lava that fall around the vent of a single basaltic or andesitic eruption.

Cirque

A bowl-shaped, amphitheater-like depression eroded into the head or the side of a glacier valley. Typically, a cirque has a lip at its lower end. The term is French and is derived from the Latin word circus.

Coulee

An elongate lavadome that typically forms as a high-silica lava erupts on the steep flank of a volcano. The dome axis perpendicular to the volcano's slope is shorter than the axis parallel to the slope, creating an elliptical shape. The leading edge of the coulee is often blocky and steep.

Crust

The crust is the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.

Cryptodome

A body of magma that rises from depth and intrudes into the edifice of a volcano, but does not erupt on the surface. Cryptodome formation can results in a bulge or welt on the surface of a volcano.

Cumulo-volcano

Dacite

Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.

Debris avalanche

Moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope.

Debris dam

Volcanic material deposited in a valley to form unstable natural dams that block pre-existing drainages; lakes may form behind the impoundment.

Deformation

Changes to the surface of a volcano that occur due to magma movement underneath the surface. Most volcano deformation can only be detected and measured with precise surveying techniques such as with a Global Positioning System (GPS), tiltmeter, Interferometric Synthetic Aperture Radar (InSAR), or an Electronic Distance Meter (EDM).

Diamict

A very poorly sorted sediment typically associated with glacial deposits.

Effusive eruption

Ejecta

Emission

The release of volcanic gases from the earth, which escape into the atmosphere.

Eruption column

The ascending, vertical part of the mass of erupting debris and volcanic gas that rises directly above a volcanic vent. Higher in the atmosphere, columns usually spread laterally into plumes or umbrella clouds.

Explosive eruption

Extensional tectonic environment

A region of the earth's crust or lithosphere that is spreading apart due to tectonic forces acting on the surrounding areas. Back-arc basins are regions of extension behind subduction zones and are often locations where volcanoes form.

Facies

A body of rock with characteristics that are the same. Typically the facies is a unit of rock that was formed under conditions reflecting a specific emplacement process e.g., a single lava flow, an ash-flow tuff, a lahar.

Fallout

A general term for all the ash and debris that falls to earth (also known as ashfall) from an eruption cloud.

Fissure

In geology, a fissure is a fracture or crack in rock along which there is a distinct separation; fissures are often filled with mineral-bearing materials. On volcanoes, a fissure is an elongate fracture or crack at the surface from which lava erupts.

Igneous

Intrusive

Molten rock that is forced into pre-existing rocks, or an igneous rock that has crystalized (turned to solid) below the earth's surface.

Isopach

A line on a map that connects geologic units of equal rock thickness. In volcanology, this typically relates to the thickness of an ash or tephra deposit from an explosive eruption.

K-Ar

Symbol for potassium-argon; typically used when referring to the potassium-argon age dating method, which measures the radioactive decay of potassium into argon and provides an absolute age for rock samples older than a few thousand years.

ka

Thousand years ago.

Lahar

Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows.

Lateral Blast

A lateral (sideways) explosion with a significant low-angle component that is directed towards an area that can cover as much as 180 degrees. Because they carry rock debris at high speeds, lateral blasts can devastate areas tens to hundreds of square kilometers within a few minutes, and they can destroy manmade structures and kill all living things by abrasion, impact, burial, and heat.

Lava

General term for magma (molten rock) that has been erupted onto the surface of the Earth and maintains its integrity as a fluid or viscous mass, rather than exploding into fragments.

Lava flow

Streams of molten rock that erupt relatively non-explosively from a volcano, then move downslope until they stop, cool, and solidify.

Lava fountain

A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is called a lava fountain. Lava fountains typically range from about 10 to 100 m in height, but occasionally reach more than 500 m.

Lava lake

Large volume of molten lava, usually basaltic, in a vent, crater, or broad depression, which forms a lake.

Lava tube

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels and pahoehoe flows.

Lithic

In volcanology: rocks that are not from the primary eruptive magma, but are instead pulled from the walls of the magma storage zone or conduit during an eruption. Previously formed rocks not from the primary magma source.

Littoral cone

A cone of lava fragments built on the surface of a lava flow pouring into a body of water, usually the sea, is called a littoral cone ("littoral" refers to a shoreline).

Ma

Million years ago.

Maar

A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Maars often fill with water to form a lake.

Moat

Monogenetic

Resulting from one process or formation or derived from one source, or originating or developing at one place and time. e.g. a volcano built up by a single eruption.

Obsidian

Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian forms in lava flows where the lava cools so fast that crystals do not have time to grow.

Ophiolite

An assemblage of crustal rocks that represents a section of the Earth's oceanic crust and upper mantle that has been uplifted and exposed above sea level.

PAHOEHOE

Basalticlava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.

Paleomagnetic studies

Investigations of the orientation and(or) intensity of the Earth's magnetic field in the past, as recorded in geologic materials. The magnetic poles wander about the Earth's axis of rotation, and the paleomagnetic pole position at the time of cooling of a volcanic rock is "frozen" by magnetic minerals. An empirical calibration of this "secular variation" over time allows eruption ages to be constrained and isolated outcrops to be correlated with one another.

Petrologic

Petrology is the study of rocks, including their occurrence, composition, and origin.

Phreatic eruption

An eruption that primarily involves steam explosions, usually ground water flashed into steam by the heat of subsurface magma.

Phreatomagmatic eruption

An eruption that involves both magma and water, which typically interact explosively, leading to concurrent ejection of steam and pyroclastic fragments.

Pillow lava

Mounds of elongate lavapillows formed by repeated oozing and quenching of hot basalt.

Pleistocene

The first epoch of the Quaternary period (the most recent geologic time period) that lasted from 2,588,000 to 11,700 years ago, spanning the world's recent period of repeated glaciations. It is the period of time before the Holocene.

Plinian

Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuated areas around the volcano, but about 2,000 were killed, including Pliny the Older.

Postglacial

Refers to the time since the end of the last major ice age - this varies by location.

Pumice

Highly vesicular volcanic ejecta, typically silicic in composition. It is essentially magma that has been frothed up by escaping gases and then cooled and solidified during eruption. Rhyoliticpumice is typically of low enough density that it floats on water. Near a vent, hot pumice can accumulate and form a pumice cone.

Pyroclast

General term applied to volcanic products or processes that involve explosive ejection and fragmentation of erupting material. Literally means "fire-broken."

Pyroclastic flow

A hot (typically >800 °C), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front.

pyroclastic surge

Ground-hugging clouds of ash, rock, and volcanic gas that move at hurricane velocities and have temperatures of several hundred degrees Celsius.

Quaternary

The most recent geologic time period and spans 2,588,000 years ago until the present.

Resurgent dome

The central highland in many large calderas formed by gradual upwarping of the caldera floor after caldera collapse as a result of renewed magma intrusion.

Rhyodacite

Rhyolite

Volcanic rock (or lava) that characteristically is light in color, contains 69 or more percent of silica, and is rich in potassium and sodium. Low-silica rhyolite contains 69 to 74 percent silica. High-silica rhyolite contains 75 to 80 percent silica. Rhyolitic lavas are viscous and tend to form thick blocky lava flows or steep-sided piles of lava called lava domes. Rhyolitemagmas tend to erupt explosively, commonly also producing abundant ash and pumice.

Scoria

Vesicular volcanic ejecta, essentially magma that has been frothed up by escaping gases. It is a textural variant of pumice, with scoria typically being less vesicular, denser, and usually andesitic or basaltic.

Sedimentary rocks

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.

Seismic swarm

A series of minor earthquakes occurring in the same area and time, none of which may be identified as the main shock or with the same fault.

Seismic tomography

Seismic tomography is similar to a CAT scan. When earthquakes occur, they sent out seismic waves, and by recording the patterns of the wave arrivals from many earthquakes on many seismometers, seismologists can calculate the wave speeds through the crust. Fast and slow wave speeds can then be interpreted to geologically understand the tectonic and/or volcanic structure of the earth.

Seismicity

The phenomenon of earthquakes caused by the brittle fracturing of rocks in the earth's crust. Synonymous with seismic activity.

Shield volcano

A broad shield-shaped volcano that is built up by successive, mostly effusive, eruptions of low-silicalava.

Silica

Silicon dioxide, the most abundant rock-forming compound on Earth and the predominant molecular constituent of volcanic rocks and magmas. It tends to polymerize into molecular chains, increasing the viscosity of the magma. Basaltic magma, having lower SiO2, is fairly fluid, but with increasing contents of SiO2, andesite, dacite, and rhyolite magmas become progressively more viscous. Because it is more difficult for dissolved gas to escape from more viscous magma, higher silica magmas generally erupt more explosively.

Silicic

Sinter

Also known as siliceoussinter. The lighweight, porous, opaline variety of silica that is white or nearly white and deposited as an incrustation by precipitation from the waters of geysers and hot springs.

Spatter cone

A steep-sided cone constructed of agglutinate at a maficvent. Most spatter cones are small (typically 10 m or less in height) and commonly form in linear groups along a fissure.

Stratigraphy (Stratigraphic)

The science of rock strata, or layers. It is concerned with all characters and attributes of rocks as sequentially timed layers and their interpretation in terms of mode of origin and geologic history. The arrangement of strata signifies chronologic order of sequence.

Submarine volcano

Tectonic

Tephra

Any type and size of rock fragment that is forcibly ejected from the volcano and travels an airborne path during an eruption (including ash, bombs, and scoria).

Tertiary

Geologic period from 65 million to 2.6 million years ago. This time period began with the extinction of the non-avian dinosaurs and signifies the end of the Cretaceous Period and the start of the Cenozoic era.

Thermal

Of or relating to heat. At volcanoes, thermal features are observed to determine whether temperatures are changing (e.g. fumaroles, vents, lava surfaces, etc...). These changes help scientists to understand volcanic processes.

Tsunami

A gravitational sea wave produced by a large-scale, short-duration disturbance of the ocean floor, such as a submarine earthquake, slump, subsidence, or volcanic eruption.

Tuff

A general term for all consolidated (hardened and/or compacted) pyroclastic (explosive, volcanic origination) rocks. Synonymous with tuffaceous.

Tumulus

A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward. The surfaces of pahoehoe flows on flat or gentle slopes often exhibit elliptical, domed structures called tumuli.

Unconsolidated

A unit of rocks that are loosely arranged or not layered, or whose particles are not cemented together, occurring either at the surface or at depth.

Vent

Any opening at the Earth's surface through which magma erupts or volcanic gases are emitted.

Volcaniclastic

A body of rock that is composed of fragments of volcanically derived rocks or minerals that were then transported some distance from their place of origin.